1. Field of the Invention
This invention relates generally to implantable devices, such as an expandable intraluminal prosthesis, one example of which includes a stent. More particularly, the invention is directed to an apparatus and method for coating a prosthesis.
2. Description of the Related Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against the atherosclerotic plaque of the lesion to remodel the vessel wall. The balloon is deflated to a smaller profile to allow the catheter to be withdrawn from the patient""s vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an expandable intraluminal prosthesis, one example of which includes a stent, is implanted in the lumen to maintain the vascular patency. Stents are scaffoldings, usually cylindrical or tubular in shape, which function to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed for insertion through small cavities via small catheters, and expanded to a larger diameter once at the desired location. Examples in patent literature disclosing stents which have been successfully applied in PTCA procedures include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
To treat the damaged vasculature tissue and assist prevention of thrombosis and restenosis, there is a need for administrating therapeutic substances to the treatment site. For example, anticoagulants, antiplatelets and cytostatic agents are commonly used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. To provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered at a specific site in comparison to larger overall dosages that are applied systemically. Local delivery produces fewer side effects and achieves more effective results.
One commonly applied technique for the local delivery of a drug is through the use of a polymeric carrier coated onto the surface of a stent, as disclosed in U.S. Pat. No. 5,464,650 issued to Berg et al. Berg disclosed applying to a stent body a solution which included a specified solvent, a specified polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend. The solvent was allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer. As indicated by Berg, stents were immersed in the solution 12 to 15 times or sprayed 20 times.
The immersion method of coating a stent, also called dip-coating, entails submerging the entire stent, or an entire section of the stent, in a polymer solution. Similarly, spray-coating requires enveloping the entire stent, or an entire section of the stent, in a large cloud of polymeric material. One disadvantage of dip-coating and spray-coating methods is the inability to control the exact geometrical pattern of coating on the stent or section of the stent. Another shortcoming of both dip- and spray-coating is the possibility of forming web-like defects by build-up of excess polymeric material between the stent struts. Web-like defects are most prevalent in stents having tight patterns, for example coronary stents, such that the distance between the struts is very small.
Another disadvantage of both dip-coating and spray-coating stems from a low-viscosity requirement for the polymer solution in which the stent is dipped or with which the stent is sprayed. A low viscosity solution can only be achieved by using a low molecular weight polymer or by using a very low concentration of polymer in the polymer solution. Thus, both dip-coating and spray-coating methods have imposed limitations in type and concentration of applied polymers.
Other commonly applied techniques for coating a stent with a polymeric material include sputtering and gas phase polymerization. Sputtering typically involves placing a polymeric coating material target in an environment, and applying energy to the environment that hits the target and causes emission of polymeric material from the target. The polymeric emissions deposit onto the stent, forming a coating. Similarly, gas phase polymerization typically entails applying energy to a monomer in the gas phase within a system set up such that the polymer formed is attracted to a stent, thereby creating a coating around the stent.
Sputtering and gas phase polymerization have similar shortcomings. Like the dip-coating and spray-coating techniques, the sputtering and gas phase polymerization techniques do not allow control of the geometrical pattern of the coating and are quite limited in the selection of polymers that can be employed. In addition, coating a stent with a polymer and a drug at the same time via sputtering or gas phase polymerization has not been demonstrated to be effective and risks degradation of the drug. Moreover, techniques for applying a polymeric coating by sputtering or gas phase polymerization and later incorporating a drug into the applied polymeric coating are limited.
Accordingly, it is desirable to provide an improved method of applying a polymeric coating to a prosthesis. Specifically, it is desirable to provide a method of applying a polymeric coating to a prosthesis which enables control over the geometrical pattern in which a prosthesis is coated, reduces the incidence of web-like defects due to excess build-up of polymeric material, broadens the field of both the types and the concentrations of polymers which may be used to coat a prosthesis, and allows a prosthesis to be coated with a polymer and a drug at the same time.
In accordance with one embodiment of the present invention, a method of forming a coating onto a surface of a prosthesis, such as a stent, is provided. The method comprises providing a composition and depositing the composition in a preselected geometrical pattern onto a first surface of the prosthesis to form the coating.
In one embodiment, the method comprises providing a composition that includes a polymer and a solvent. The polymer can constitute from about 0.1% to about 25% by weight of the total weight of the composition and the solvent can constitute from about 75% to about 99.9% by weight of the total weight of the composition.
In accordance with another embodiment, sufficient amounts of a therapeutic substance or a combination of substances are included in the composition of the polymer and the solvent. In this embodiment, the polymer can constitute from about 0.1% to about 25% by weight of the total weight of the composition. The solvent can constitute from about 49.9% to about 99.8% by weight of the total weight of the composition. The therapeutic substance can constitute from about 0.1% to about 50% by weight of the total weight of the composition.
In accordance with other embodiments, the method comprises providing a composition that includes a monomer. A monomeric composition may also include a solvent and/or a therapeutic substance. The monomeric composition may be cured to form a polymeric coating.
In accordance with other embodiments, the method comprises providing a composition that includes a polymer without a solvent. The composition may also include a therapeutic substance. The composition may be heated prior to being deposited onto the prosthesis.
In accordance with other embodiments, the method comprises providing a composition that includes a therapeutic substance. The composition may also include a solvent.
In accordance with one embodiment, depositing the composition in a preselected geometrical pattern comprises moving a dispenser assembly along a predetermined path while depositing the composition onto a stationary prosthesis. In accordance with another embodiment, depositing the composition in a preselected geometrical pattern comprises moving a holder assembly supporting the prosthesis along a predetermined path while a stationary dispenser assembly deposits the composition onto the prosthesis. In accordance with still another embodiment, depositing the composition in a preselected geometrical pattern comprises moving a holder assembly supporting the prosthesis along a first predetermined path and moving a dispenser assembly along a second predetermined path.
The preselected geometrical pattern of the composition as deposited onto a surface of the prosthesis may be a continuous stream that is either in a substantially straight line or a line that has a curved or angular pattern. The preselected geometrical pattern may also be an intermittent pattern that is in a straight line, a line that is curved or angular, or includes at least one bead.
In accordance with some embodiments, the prosthesis contains a channel extending from a first position along the first surface to a second position along the first surface and within which the composition is at least partially deposited. The preselected geometrical pattern of the composition as deposited within a channel of the prosthesis may be a continuous stream that is in a straight line or a non-straight line such as a curved line or angular line. The preselected geometrical pattern may also be an intermittent pattern that is in a straight line, a non-straight line such as a curved line or angular line, or includes at least one bead.
In accordance with other embodiments, the prosthesis contains a first cavity within the first surface of the prosthesis within which the composition is at least partially deposited. The predetermined geometrical pattern may be a bead.
In some embodiments, the application of the composition to the prosthesis is followed by the redistribution of the composition along the prosthesis. Redistribution of the composition may be accomplished by using, for example, air pressure, centrifugal force, or a second solvent.
The polymer, with or without the therapeutic substance, solidifies and adheres to the prosthesis following removal of the solvent to substantial elimination.
In accordance with another embodiment of the invention, an apparatus for depositing a composition onto a surface of a prosthesis is provided. The apparatus comprises a dispenser assembly having a nozzle for depositing a composition onto a surface of a prosthesis, a holder assembly for supporting a prosthesis, and a motion control system for either moving the dispenser assembly along a predetermined path or moving the holder assembly along a predetermined path.
The dispenser assembly may deposit the composition in a preselected geometrical pattern onto a surface of the prosthesis, at least partially within a channel formed into the prosthesis or at least partially within one or more cavities formed into the prosthesis. The preselected geometrical pattern of the deposited composition may be a continuous stream that is in a straight line or a non-straight line such as a curved line or angular line. The preselected geometrical pattern may also be an intermittent pattern that is in a straight line, a non-straight line such as a curved line or angular line, or includes at least one bead.
In accordance with some embodiments, the dispenser assembly can deposit a second composition in a preselected geometrical pattern onto the prosthesis. The first and second compositions may be in contact with one another in at least one location on the prosthesis.
In some embodiments, the dispenser assembly has a nozzle having an orifice with an orifice diameter in the range of approximately 0.5 microns to approximately 150 microns. In other embodiments, the nozzle has an orifice that can capture a last droplet of the composition to prevent lifting of the last droplet from the prosthesis. In other embodiments, the nozzle can be positioned at a 90xc2x0 angle with respect to the prosthesis during deposition of the composition. In still other embodiments, the nozzle can be positioned at an angle less than 90xc2x0 with respect to the prosthesis during deposition of the composition. In other embodiments, the dispenser assembly has more than one nozzle.
In accordance with some embodiments, the dispenser assembly is coupled to a delivery control system. The delivery control system may be in communication with a CPU.
In accordance with some embodiments, the motion control system is for moving the dispenser assembly along a predetermined path. The motion control system may be in communication with a CPU and may move the dispenser assembly along a predetermined path in the x, y, z, and/or rotational directions.
In accordance with other embodiments, the motion control system is for moving the holder assembly along a predetermined path. The motion control system may be in communication with a CPU and may move the holder assembly along a predetermined path in the x, y, z, and/or rotational directions.
In accordance with other embodiments, a first motion control system is for moving the dispenser assembly along a first predetermined path and a second motion control system is for moving the holder assembly along a second predetermined path.
In accordance with some embodiments, the apparatus additionally includes a feedback system. The feedback system includes a video camera for capturing an image, a lens system coupled to the video camera, frame grabber hardware to accept the image, and vision software to characterize the image. Image data from the video camera is fed back to the motion control system, the dispenser assembly, and/or the holder assembly to direct deposition of the composition onto the surface of the prosthesis.